Methods of measuring relative humidity include mechanical and electrical methods. The latter is advantageous over the former in points of having the possibility of size reduction of instrument, rapid response, and easy conversion of humidity to electric signals. There is used an electric resistance type humidity-sensitive element utilizing the humidity-sensitive property of various materials in the electrical humidity measuring method. This humidity-sensitive element is constructed from an insulating substrate, a pair of electrodes arranged thereon, and a layer of humidity-sensitive material covering these electrodes.
The humidity-sensitive material conventionally used in such electric resistance type humidity-sensitive element is as follows:
(1) Ceramic materials. PA0 (2) Electrolytes such as lithium chloride (LiCl). PA0 (3) Hygroscopic resins containing dispersed conductive powder. PA0 (4) Hydrophilic polymers or polyelectrolytes. PA0 (5) Hydrophobic materials which is made hydrophilic by introducing cationic group into the molecule of hydrophobic polymers.
These types of humidity-sensitive materials had the following disadvantages, respectively:
Type (1): The moisture adsorption on these ceramic materials is partly due to chemisorption which is irreversible. Accordingly, the humidity-sensitive elements employing these humidity-sensitive materials exhibit significant hysteresis and poor response, and additionally tend to cause deformation on standing in a high humidity atmosphere for a long period of time.
Type (2): These materials cause deliquescence in a high humidity atmosphere and eventually flow out. Accordingly, the humidity-sensitive element employing these materials have extremely short life spans, and the range of humidity measurable for a single element of this type is narrow.
Type (3): These materials exhibit no humidity-sensitive property in a low humidity atmosphere. It is difficult to make the degree of dispersion of conductive powders uniform, and hence this type of humidity-sensitive element results in a low yield of production and is also poor in reliability.
Type (4): These materials are better than the above three types of material in humidity-sensitive property, but cause expansion or shrinkage depending upon the moisture content in the surrounding atmosphere because they absorb large amounts of moisture. For this reason, a layer of this type of materials used is liable to peel from the substrate or the electrode.
Type (5): These materials, disclosed in Japanese Patent Application Laid-open No. 80191/1979, have been developed in order to solve the problems involved in the above four types of materials. It is desirable to hold a linear relationship between the relative humidity values to be detected by this type of element and the logarithms of the corresponding electric resistances of the element, since the relative humidity is converted to electric signals on the basis of the relation between these two parameters. However, no linear relationship exists between them in humidity sensing element employing humidity-sensitive materials disclosed in said patent application, and additionally considerable hysteresis is exhibited thereby. This complicates the conversion circuit and produces large errors between measured and true humidity values.
It is further desired to reduce the electric resistance between electrodes, since a low value of the resistance reduces the disturbing effect of noises and as a result higher precision measurement can be expected.